Plants for the generation of electrical energy, in particular steam power stations, are conventionally designed for operating with a specific power output, the nominal power output, so that, when the plant is operating with this power output, optimum operating conditions of the numerous plant components are obtained, for example in terms of wear, frictional forces and frictional losses which occur, the generation of noise, exhaust gas behavior and efficiency.
In known power plants, there is often the problem that demand-related load changes cannot be carried out as quickly as desired while the power plant is in operation. For example, the speed of load change of steam power stations is restricted by the temperature variations occurring in one or more power station components as a result of a load change, in particular by the temperature variations in thick-walled plant components in which the temperature effects mentioned are particularly pronounced. Temperature variations of this kind have, inter alia, an adverse effect on a desired speed of load change which is as high as possible, since the temperature gradients which arise generate, in addition to the mechanical stresses prevailing in the affected plant component or plant components and caused, for example, during operation, further mechanical stresses in the material from which the plant component is manufactured. These additional stresses, caused by the temperature gradients mentioned, contribute to the fatigue of the material, so that the strength of the latter may decrease or else damage to the plant component is to be feared.
The problem mentioned arises particularly in the case of power plants with a high power output, which are designed as steam power stations and are equipped with a steam boiler which is operated by natural or forced circulation. The power plants mentioned comprise, as a rule, thick-walled drums for steam separation. In this case, in particular, the material of the steam separation drum is put at risk in the event of too rapid a load change as a result of the temperature gradients occurring under these circumstances, so that power plants of this type have hitherto been designed for operating in a constant-pressure regime, in order to avoid pressure and/or temperature fluctuations to which the steam separation drum is exposed. Such power plants known from the related art are therefore operated in the part-load range by a throttling of the turbine valves and/or by only partial action of operating steam on a first turbine stage, so that the pressure conditions in the part-load range are consequently comparable to the pressure conditions in the nominal-load range and the desired constant-pressure regime is thus obtained.
Such a throttling of the turbine valves, which is necessary during the entire operating time in the part-load range, brings about an appreciable loss of efficiency of the power plant, as compared with the efficiency of this plant which is achievable in the nominal-load range.
When the first turbine stage is acted upon only by part of the operating steam (partial action) in order to operate the power plant in the part-load range, this requires a special and complicated form of construction of the turbine, in which a regulating device, for example a regulating wheel, then has to be present in order to implement the possibility of partial action. Such a form of construction of the turbine is highly complicated in structural terms and is often susceptible to faults in operational terms.